In recent years superconductor materials have been the subject of increasing interest and research. Although relatively high superconducting transition temperatures (above 23 K) have been achieved using some oxide materials, such materials are often extremely complex and expensive to produce. It would therefore be desirable to develop alternative superconductive materials, which preferably are less complex and less expensive to make.
Carbon fullerenes (hereinafter referred to as "fullerenes"), sometimes referred to as buckyballs or buckminsterfullerenes, are a relatively newly discovered class of molecular materials. The most commonly discussed fullerenes are C.sub.60 and C.sub.70. A C.sub.60 fullerene molecule consists of 60 carbon atoms joined together to form a cage structure with 20 hexagonal and 12 pentagonal faces symmetrically arrayed in a soccer ball-like structure. C.sub.60 molecules form a close-packed solid molecular material having a face-centered cubic structure. The structure of C.sub.70 has 25 hexagons, resulting in a shape reminiscent of a rugby ball.
Various attempts have previously been made to produce superconductive fullerene materials. For example, "Conducting Films of C.sub.60 and C.sub.70 by Alkali-Metal Doping", by Haddon et al., Nature, Vol. 350, No. 6316, pp. 320-322 (March, 1991), discloses alkali-metal doped C.sub.60 films which are superconductive.
"Superconductivity in Doped Fullerenes", by Hebard, Physics Today, November 1992, pages 26-32, discusses physical properties and theoretical mechanisms of superconductivity in alkali-metal doped C.sub.60 fullerene molecules, and states that the maximum T.sub.c obtained was 33 K for RbCs.sub.2 C.sub.60.
"Fullerenes", by Curl et al., Scientific American, October 1991, pages 54-63, discloses C.sub.60 fullerene molecules and teaches that, when doped with potassium or rubidium, fullerene molecules become superconductive. When rubidium is used, for example, the critical temperature for superconductivity was found to be near 30 K.
"Solid C.sub.60 ", by Huffman, Physics Today, November 1992, pp. 22-29, discusses methods for doping C.sub.60 with alkali-metal ions and thallium, and reports that the highest T.sub.c obtained was 42.5 K for C.sub.60 with rubidium-thallium doping.
Unfortunately, due to the extreme sensitivity of alkali metal-containing materials to oxidation, alkali-metal-doped fullerenes are very unstable in air. Consequently, these materials are very difficult to work with, and lose their superconductive properties within minutes of being exposed to oxygen. It would, therefore, be desirable to find alternative methods and materials for doping fullerenes.
"Fluorination of Buckminsterfullerene" by Holloway et al., J. Chem. Soc., Chem. Commun., pages 966-969 (1991) discloses contacting solid C.sub.60 with fluorine gas to produce C.sub.60 F.sub.60. However, this material is touted for its possible lubricating properties, and there is no mention or suggestion of this material being superconductive.
"Intercalation of Solid C.sub.60 with Iodine", by Zhu et al., Nature, Volume 355, page 712-714 (Feb. 20, 1992), discloses the doping of solid C.sub.60 with iodine to produce C.sub.60 I.sub.4, and states that there is no superconductivity observed in this material down to 4 K.
There is generally an ever present need for alternative superconductive materials, particularly those which show promise for exhibiting high (i.e., above 23 K) superconductive transition temperatures. As for fullerene based superconductive materials, the need for higher superconductive transition temperatures remains.